Ink jet printing systems are one example of digitally controlled fluid ejection devices. Ink jet printing systems are typically categorized as either drop-on-demand printing systems or continuous printing systems.
Drop-on-demand printing systems incorporating a heater in some aspect of the drop forming mechanism are known. Often referred to as “bubble jet drop ejectors” or “thermal ink jet drop ejectors”, these mechanisms include a resistive heating element(s) that, when actuated (for example, by applying an electric current to the resistive heating element(s)), vaporize a portion of a fluid contained in a fluid chamber creating a vapor bubble. As the vapor bubble expands, liquid in the liquid chamber is expelled through a nozzle orifice. When the mechanism is de-actuated (for example, by removing the electric current to the resistive heating element(s)), the vapor bubble collapses allowing the liquid chamber to refill with liquid.
In a thermal ink jet printing device, there are typically hundreds of thermal ink jet drop ejectors which are grouped into one or more arrays. Large numbers of drop ejectors are useful for a high degree of addressability for high resolution printing, as well as for high throughput printing. In a color printing system, different arrays of drop ejectors are typically used to print at least cyan, magenta and yellow ink.
Thermal ink jet printheads may be classified as either face-shooting devices or edge-shooting devices. In both types of configurations the resistive heating elements are formed, typically together with driving and addressing electronics, at or near the planar surface of a substrate such as a silicon die. In a face-shooting device, the drop of liquid is ejected perpendicular to the plane of the substrate. Face-shooting devices include both roofshooters and backshooters. In a roofshooting device the direction of ink ejection is the same as the direction of bubble growth. In a backshooter, the direction of ink ejection is opposite the direction of bubble growth. In an edge-shooting device, the drop is ejected in a direction which is substantially parallel to the plane of the substrate. In a face-shooting device nozzle orifices may be readily formed in a two-dimensional configuration. In an edge-shooting device the orifices are typically arranged within a single line along the edge of the device.
Within a high resolution, high throughput printer there may be a plurality of printheads or silicon substrates to provide the multiple nozzle arrays that are needed. For example, in a color printer there may be four separate printheads for printing cyan, magenta, yellow and black inks. For excellent image quality, it is necessary to align the corresponding spots from different arrays. For the case of separate printheads, it is generally necessary to perform a subsequent alignment for suitable image quality. Some of the alignment is typically done mechanically, for example by physical contact of the printheads with reference surfaces provided within the printer. Electronic compensation for printhead misalignment may also be done in the printer. For example, a print test pattern may be used in order to select which nozzles from the different arrays should correspond to one another for best alignment, and in order to set the relative timing of the firing of the printheads.
One solution for alignment of different arrays of nozzles is to fabricate all of the arrays on the same silicon die. U.S. Pat. No. 5,030,971 describes a printhead having a heating element substrate with at least two ink inlets and corresponding arrays of nozzles and their associated heating elements. In such a configuration, the ink inlets may be used such that each feeds a different color of ink. In a different application they may all feed a single ink color. In addition, the nozzles on either side of an ink inlet may be staggered with respect to each other so that double the addressable printing resolution is provided. '971 also discloses that if the plurality of ink inlets feed the same type of ink, and if the nozzle arrays are also offset by a fraction of the nozzle spacing with respect to each other, then even higher addressable printing resolution is possible.
An approach similar to '971 of providing multiple staggered linear arrays of nozzles for high single pass printing resolution is also described in U.S. Pat. No. 6,543,879.
Arrays which are formed on the same silicon die are made with the high precision inherent in photolithography and microelectronic fabrication processes, which provides sufficient alignment. However, in some applications, forming all of the required arrays on one die may cause the die size to grow so large that it is too costly.
One alternative is to bond a plurality of silicon die to a common support member. The relative alignment between arrays on different die which are bonded to the same substrate is not as precise as within a single die (e.g. within 1 micron), but a fairly high degree of alignment precision (e.g. within 10 microns) may still be built into the printhead using such an approach.
An example of bonding a plurality of thermal ink jet die onto a common support member is a pagewidth array. Most thermal ink jet products at present are carriage-style printers and are comprised of die with printing array lengths of about 1 to 3 cm. These arrays are typically scanned across the paper (substantially perpendicular to the array length) in order to print a swath. Then the paper is advanced in a direction parallel to the array length so that the printheads can print the next swath. In a pagewidth array printer, drop ejection nozzles are provided across the entire width of a page, so that it is not necessary to have relative movement between the printhead and paper along the direction of the array length. Due to fabrication yield, it may be prohibitively expensive to make high quality printing arrays which are comprised of a single die, which would need to be at least 20 cm long. Instead, a pagewidth printhead is assembled by bonding a plurality of die on a common support member. For pagewidth printheads the N die are positioned such that the combined array length is approximately N times the array length on a given die. The die may be positioned end to end, or in staggered fashion. For the staggered configuration, some overlap of the printing areas of neighboring die is possible, so that the overall array length is a little less than N times the individual array length.
For some carriage-style printer applications it is also advantageous to bond multiple die to the same support member. U.S. Pat. No. 6,659,591 describes the construction of a printhead having a first roofshooting die with ink inlets and ejectors for cyan, magenta and yellow ink, and a second roofshooting die with ink inlet and ejectors for black ink. Both die are bonded to the same support member. In such a printhead, the die are typically bonded with the nozzle arrays substantially parallel with one another, rather than in end-to-end fashion. The motivation for multiple die on a substrate in such an application is compactness of the printing unit, as well as some degree of built-in precision alignment.
In some printing applications it is useful to have different groups of drop generating elements, such that each group is designed to eject droplets of a particular drop size. The nominal drop volume for a given thermal ink jet drop ejector depends mainly on design parameters such as heater area, nozzle orifice area and chamber geometry, and also somewhat upon properties of the fluid being ejected. Thermal ink jet drop generators are capable of providing only a somewhat limited range of variation of drop size by methods such as modifying the current pulse train to the resistive heating elements. Therefore in applications where it is desired to do gray scale printing by deposition of different volumes of ink on each pixel site, it is useful to provide a plurality of nozzle arrays such that the drop generators in each array prints a given drop volume, which is different from the drop volume ejected by drop generators in a different array. U.S. Pat. No. 4,746,935 discloses a printhead where three drop generators in a row are weighted to provide drop volumes in a ratio of 1:2:4. The row of different sized drop generators is parallel to the scanning direction of the printhead during printing, so that by proper timing of the firing, droplets from each of the three different sized drop ejectors can land in the same location on the paper. Different combinations of drop sizes printed on the same pixel site can provide up to 8 different levels of ink coverage.
U.S. Pat. No. 5,412,410 discloses an edge-shooter type thermal ink jet printhead in which two groups of nozzles are collinearly arranged where the nozzles from first group are equally spaced in alternating fashion with nozzles from the second group. Nozzles from the two groups produce different drop sizes. By proper timing of the firing of the second group of nozzles relative to the first group, it is possible to position small drops at the interstices between large drops using such a nozzle configuration. In the configuration disclosed, the small drops would be the same ink type as the large drops. A disadvantage of multiple groups of nozzles arranged on an edgeshooter is that the nozzle resolution is limited by the requirement that all of the nozzles be arranged in a single line.
U.S. Pat. No. 6,592,203 discloses a printhead having a line of nozzles of one size disposed in alternating fashion with a second line of nozzles which is parallel to the first line of nozzles and having a different nozzle size. In the method of printing which is disclosed in this patent, columns of pixel locations are arranged on the print media. In a first set of columns of pixel locations, a large dot of a given ink type may be printed in the first pixel location. In a second set of columns of pixel locations, which are interleaved with the first set of columns, a small dot of the same ink type would be available to be printed. This is made possible by gearing the paper advance with a resolution of double the resolution of the nozzles.
As discussed above, in a printing system it is sometimes advantageous to provide different sized drop ejectors so that at least one ink may be selectively ejected with different drop volumes. In addition, it is sometimes useful to provide different sized drop ejectors corresponding to the different liquids that are being ejected. Some ink types have different spreading properties on the print media than others. For example, color inks are sometimes designed to penetrate rapidly into uncoated papers (so that adjacent printed colors do not bleed into one another), while the black ink may be designed to penetrate slowly into such papers. This allows the black ink to spread more controllably, without undesirable wicking along paper fibers, so that black text can be clear and crisp. In such a printing system, it would be desirable for the black drop ejectors to eject a larger drop volume than the color drop ejectors in order to enable full coverage of the paper.
U.S. Pat. No. 5,570,118 discloses a color printing system in which two different black inks are printed with two different printheads. The first black printhead ejects ink having a high surface tension (greater than 40 dynes/cm) so that it does not spread rapidly and is suitable for sharp edges on lines and text. This first black printhead is separated by a small gap from a set of secondary printheads for ejecting cyan, magenta, yellow and a second type of black ink. Each of the inks in the secondary printheads has a surface tension less than 40 dynes/cm. Low surface tension inks tend to penetrate into the paper more rapidly and are less likely to bleed into adjacent regions of printed ink of a different color. The intent is to use the secondary printheads for printing color portions of the image, and the first black printhead for printing portions of the image containing only black. One drawback of this configuration where the two different arrays of black drop ejectors are on separate printheads is that it is difficult to align the separate printheads such that the spots from different black arrays are precisely positioned with respect to one another with an alignment error of less than one pixel spacing.